Reversed-phase high-performance liquid chromatography of ionogenic compounds: comparison of retention models

Two kinds of retention models describing a behaviour of ionogenic substance s in reversed-phase chromatographic systems were compared. Model A utilises a concept of limiting retention factors and is especially suitable for the prediction of retention of compounds co-existing in several forms in mobil e phase. An effect of the concentration of organic modifier (e.g., methanol ) on the magnitudes of the limiting retention factors and equilibrium const ants (dissociation constants of the separated substances) can be expressed with the aid of various, more or less sophisticated, relationships. A stoic hiometric displacement model (model B) in its original form simply relates the analyte retention to the content of organic modifier in the mobile phas e. In this work, it was modified to also express an effect of the mobile ph ase pH introducing side equilibria (acid-base) into the model. Both models predict a sigmoidal dependence of the analyte retention factor on the mobil e phase pH in accordance with experimental data, and allow among others, to estimate dissociation constants from those data. Experimental dependencies between the analyte retention and the concentration of methanol in the mob ile phase comply well with model A, whereas the stoichiometric displacement model could be used only in a limited range of the methanol concentrations . (C) 1999 Elsevier Science B.V. All rights reserved.